fix(ui): update plan descriptions to reflect per-user pricing

- Rewrite taglines and feature lists for all four tiers with
  concrete, use-case-driven copy (freelancers, knowledge workers,
  power users — no team or enterprise framing)
- Rename 'Business' display name to 'Power' (plan_id stays
  'business' for backwards DB compatibility)
- Replace 'enterprise' language in pricing page hero with
  'per person, per month' copy
- Swap fa-building icon for fa-bolt on the Power tier
- Fix support level for Power tier to 'Priority' (was 'Dedicated')
- Update docs/SubscriptionTiers.md with new names, table, and
  intended-use-case section
- Add test_business_tier_display_name_is_power assertion

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/subscription.html

@@ -42,7 +42,7 @@
           {% if tier_id == 'free' %}fa-seedling text-gray-500
           {% elif tier_id == 'starter' %}fa-rocket text-blue-600
           {% elif tier_id == 'professional' %}fa-star text-indigo-600
-          {% else %}fa-building text-purple-600{% endif %}
+          {% else %}fa-bolt text-purple-600{% endif %}
           text-2xl" aria-hidden="true"></i>
       </div>
       <p class="text-xs font-semibold text-gray-400 uppercase tracking-widest">Current Plan</p>